The present invention relates to an electric motor with permanent-magnet excitation, which has a rotor on whose surface, oriented toward the inside of a stator, a plurality of permanent magnets are disposed.
A permanent-magnet-excited electric motor of this kind is known for instance from U.S. Pat. No. 6,204,587 or German Patent DE 3 844 074 C2. The rotors of the electric motors described in these publications have permanent magnets on their surface, and adjacent permanent magnets have opposite polarity. As taught in particular by DE 3 844 074 C2, especially in high-speed motors, provisions must be made to absorb the centrifugal forces acting on the permanent magnets. To that end, a massive hollow cylinder is slipped onto the rotor, in such a way that it rests with its inner wall directly on the permanent magnets. This fixes the permanent magnets in their position on the rotor, and thus a fastening of the permanent magnets that is reliable even at very high rpm is achieved. Such relatively complicated provisions for fixation of the permanent magnets are required quite particularly whenever the permanent magnets have a great wall thickness, so that they can be magnetized not radially but laterally, so that a magnetic short circuit through the rotor can be dispensed with. Permanent magnets with a great wall thickness are typically made by pressing from a magnetic material. Pressed magnetic material has only slight mechanical tensile strength, however, so that bandaging of the permanent magnets is required as disclosed for instance in DE 3 844 074 C2.
The object of the invention is to disclose an elm of the type defined at the outset in which the magnetic material forming the permanent magnets can be applied in a relatively thin layer directly onto the surface of the rotor.
It is an object of the present invention to provide an electric motor of the above-described type having a stator and a rotor rotatably mounted in the stator, which has a plurality of permanent magnets disposed on a surface of the rotor oriented toward the stator, in which the magnetic material forming the permanent magnets can be applied in a relatively thin layer directly onto this surface of the rotor.
This object and others, which will be made more apparent hereinafter, are attained in an electric motor with permanent-magnet excitation, which comprises a stator, a rotor that is supported rotatably inside the stator and has a surface facing the stator and a number of permanent magnets arranged adjacent to each other on the rotor surface with adjacent permanent magnets magnetized in opposite directions radially with respect to the rotor surface;
wherein the rotor is provided with cavities arranged within its interior and with indentations in the rotor surface, which are placed and dimensioned so that magnetic flux lines oriented radially to the rotor surface are not interrupted by them so that no short circuit is thereby formed via the rotor, but magnetic flux lines oriented tangentially or nearly tangentially to the rotor surface are interrupted by them so that a short circuit is thereby formed via the rotor; and
wherein the cavities in the interior of the rotor are positioned below respective transitions between adjacent permanent magnets magnetized in the opposite directions, the cavities extend in a direction of a longitudinal axis of the rotor and the cavities widen transverse to the longitudinal axis of the rotor.
The coefficients of thermal expansion of the iron of the rotor and of the magnetic material in fact differ quite sharply, and the high thermal capacity of the rotor leads to rapid cooling of the magnetic material applied to the rotor, for instance by spraying. As a result, there is the danger that the applied magnetic material will tear. This unwanted effect can be avoided by the provision according to the invention for reducing the thermal capacity of the rotor, by providing cavities in the interior of the rotor. Since precisely the exciter flux, which extends radially outward from the permanent magnets of the rotor to the air gap between the rotor and the stator, increases the available torque of the motor, the recesses are placed such that they precisely do not interrupt the lines of the magnetic flux in the radial direction to the surface of the rotor.
Since the lines of the magnetic flux that are oriented tangentially or nearly tangentially to the surface of the rotor and that form a short circuit via the rotor reduce the available torque of the rotor, the cavities and indentations are placed and dimensioned such that they interrupt lines of the magnetic flux oriented tangentially or nearly tangentially to the surface of the rotor.
The permanent magnets advantageously comprise respective layers of plastic-bonded magnetic material sprayed onto the surface of the rotor next to each other in a particularly preferred embodiment of the present invention.
In another preferred embodiment the respective indentations extend in the direction of the longitudinal axis of the rotor and are formed in corresponding regions of the rotor surface below the respective transitions between the adjacent permanent magnets of opposite polarity.
The centrifugal strength of the rotor can be increased by providing that the indentations having a dovetail cross section. This is because the dovetail shape of the indentations allows wedging of the permanent magnets, so that they are secured not only against a rotary motion but also against a radial motion.
In preferred embodiments of the electric motor a nonmagnetic material for balancing the rotor, especially balancing cement, fills the cavities within the rotor interior.
The rotor advantageously comprises layered iron sheets.